Car revs fine in nuetral but wont rev under load

I bought a 2001 Escape that needed a motor. It had half a tank of gas in it once I got it running. I was told the car had been sitting for almost 2 years before I bought it. I let it idle once it was running to get the old gas out of it and make sure everything was all good. I ran it for almost 8 hours in my driveway LOL. No issues just running a little rough, and no check engine light, the gas is almost gone and I took it for a drive around the block. But it wont rev past like 2k rpms. I tried it again today in the drive way and I can floor it from a stop and it wont get over 1500 rpm in my 50ft driveway. It revs fine up to 4k or so in nuetral. Could this just be the gas?? i really hope so..

the cat looked like junk when I bolted the exhaust on I just didnt want to put a new one on til I knew it ran fine. I could see the inside was all falling apart. Im going to unbolt it and try taking it around the block once tomorrow.

The motor was bought from a junkyard so I never did anything with the timing belt. Like i said tho it revs fine when its in neutral

Also I was just thinking, would a vacuum leak cause this? I have 2 lines that have nothing on them on the back of the intake manifold. there arent any open ports on the manifold for them to go to. one is sucking and the other is coming from where the heater core lines go into the firewall

the computer is in a different section of the program when its in park.. or in drive...

there so many electronic reasons for it to do that...

do you have a scan tool../??? or somebody with one...

there are just so many things that can cause this... and the only way to know whats going on is with a scan tool... other than mechanical or fuel pressure issues... not every fault will turn on the check engine light.. and a lot of problems will prevent the motor from being damaged... as the computer will limit power ...

if you don't have a scan tool .. this is the cheepest one that actually does most of the job...

with that manual.. you will want to really look at section 5... working your way into the live data...

there are online and in print manuals available for each of the 2000+ possible trouble codes... and sometimes when swapping motors.. you have to clear the codes to get the truck or car to run properly..